NH3 (ammonia) is a corrosive, colourless gas with a sharp odour. It occurs naturally and is also manufactured by the chemical industry. Waste gas streams comprising ammonia are frequently encountered in refineries. Sometimes such waste gas streams also contain hydrogen sulphide in comparable proportions.
NH3 may be processed in a NH3 incineration process, or alternatively in a Claus process. These two routes to NH3 destruction are different in terms of equipment requirements, in chemistry and in process conditions. Both achieve the destruction of NH3. Thus, H2S gas streams containing NH3 can be employed as a feed stream in a Claus process. The Claus process is a gas desulphurising process, recovering elemental sulphur from gaseous hydrogen sulphide. Moreover, the Claus process is a very efficient process to convert NH3 in the presence of H2S and SO2 with little or no production of NO.
Downside of the Claus process is the formation of ammonium salts when a feed is used that comprises NH3 in addition to H2S. A Claus furnace that is adapted for handling the presence of NH3 is therefore run at a higher temperature (typically with increased temperature of at least 1250° C.). Moreover, the amount of oxygen required when using a mixed gas stream versus a relatively pure H2S stream is significantly higher, thereby increasing the operational and investment costs.
According to WO2006106289 a gas stream comprising hydrogen sulphide and ammonia is passed from a stripping column to a single combustion stage or furnace of a Claus plant. The combustion is conducted under conditions that eliminate essentially all the ammonia. The combustion is supported by a gas stream containing at least 40% by volume of oxygen. In WO2008124625 oxidative and reductive methods are described for the cost-effective destruction of an ammonia-containing gas stream, potentially containing minor but significant quantities of hydrogen sulphide, in a conventional Claus sulphur recovery tail gas treating unit, using controlled rates and compositions of combustion gases in order to obtain the temperatures necessary for the desired destruction of unwanted combustibles.
In U.S. Pat. No. 6,902,713 a partial oxidation procedure is described for gas containing hydrogen sulphide and ammonia in a Claus furnace with the aid of an oxygen-rich gas. The procedure involves measuring the residual content of ammonia at the output from the furnace, i.e., after the various stages of the Claus process and irrespective of the yield and conversions during the Claus process, comparing this value with a maximum value set by the user of the Claus unit, and modifying the flow of the oxygen-rich gas in proportion to the flow of ammonia gas accordingly. The residual ammonia content is measured continuously by means of a laser diode located in the main duct or a branch sampling pipe at the outlet from the Claus furnace, with the gases from the sampling pipe re-injected into the main duct, and the flow of oxygen rich gas is modified by means of a regulating loop between the continuous measuring apparatus and an automatic controller for the regulator. This sour gas stream may contain up to 60 mol % ammonia and contains significant amounts of hydrogen sulphide.
Although the above mentioned processes for treating waste streams containing hydrogen sulphide and ammonia have the advantage of eliminating essentially all the ammonia, unfortunately a Claus furnace is not always available. Moreover, a typical Claus furnace is rather complex, comprising several stages for partial combustion of hydrogen sulphide and for carrying out the Claus reaction of hydrogen sulphide with sulphide dioxide to form elemental sulphur, several condensers to recover elemental sulphur, and several reheaters to warm up the remaining gases prior to subsequent reactions. In other words, although in the aforementioned references improved Claus processes and furnaces have been described that make possible the virtual complete incineration of NH3 without NO, the downside is the investment of a relatively expensive Claus furnace and the equipment downstream of the Claus furnace, moreover run at relatively high operational costs. Finally, as is shown in U.S. Pat. No. 6,902,713, even a Claus process generally requires an incinerator for treatment of the tail gas of the Claus process.
A dedicated NH3 incinerator is more attractive than a Claus process for the treatment of waste gas streams comprising NH3 as the major or sole combustible component. However, the problem of NH3 incinerators is that oxides of nitrogen may be formed during the combustion. There is a need to destroy essentially all of the ammonia in such gas streams but without creating appreciable amounts of oxides of nitrogen in the effluent gas arising from the incineration process.
In GB2116531 a process and apparatus is described for the simultaneous disposal of NH3 containing waste gas and combustible sulphur compounds-containing waste gas. In this process combustion is carried out in three separate steps, with the combustion of the sulphur compounds-containing waste gas in the third step. In a first incineration step, the waste gas containing NH3 is combusted in the presence of a fuel gas with a first, sub-stoichiometric amount of free oxygen in the incinerator. Next, the combustion gases are mixed with a second amount of free oxygen, the total of the first and second amount being super-stoichiometric and combusting the mixture in a second incinerator. No information is provided or suggested how to optimize the combustion efficiency with further reduced NO formation.
In JP49042749 combustion of ammonia with air is described in two stages with intermediate cooling.
From EP 1106239 an alternative process is known. A gas stream containing at least 50% by volume of ammonia but less than 5% by volume of hydrogen sulphide is burned in a reaction region which is supplied with oxygen and oxygen-enriched air. Both combustion and thermal cracking of ammonia takes place in the reaction region. The rate of supplying oxygen to the reaction region is from 75 to 98% of the stoichiometric rate required for full combustion of all combustible fluids supplied to the reaction region; the ratio of oxygen to ammonia is therefore even less. The effluent from the reaction region is subsequently burned (with preferably pure oxygen or oxygen enriched air) and discharged to the atmosphere. Under these conditions essentially no ammonia remains in the effluent gas whereas formation of oxides of nitrogen can be minimised. This process therefore requires pure oxygen or oxygen enriched air. A process for incinerating NH3 that does not rely on pure oxygen or oxygen enriched air would be more attractive. Still it would be desirable to reduce the amount of NO to 100 ppm or less, preferably less than 50 ppm.
The current inventors set out to optimize the process for incinerating NH3, e.g., a stream containing at least 30 vol % NH3 and preferably containing no more than 40 vol % H2S, whereby NO formation is reduced to less than 100 ppm NO. This problem has been solved as follows.